Jul 11, 2016 UT: Photometry of SN 2016coj in NGC 4125 through cirrus

Michael Richmond
Jul 11, 2016

On the night of Jul 10/11, 2016, I observed SN 2016coj in NGC4125.

The main setup was:

Notes from the night

SN 2016coj is a Type Ia supernova in the relatively nearby galaxy NGC4125. It was discovered by the KAIT group some time before maximum light:

Here's a chart showing the galaxy, the SN, and some reference stars; the chart is about 12 x 12 arcminutes.

NGC 4125 RA = 12:08:05.7 Dec = +65:10:30 (J2000)

The AAVSO sequence team kindly provided photometry for stars near this object. You can see their full photometric sequence on their website. Below, I show only the members of that sequence which fall into my very small field of view -- taken from AAVSO sequence X16288FJI. Note that star "K" is so faint that I may not detect it clearly in B-band.

letter   B     sigB    V     sigV    R     sigR    I    sigI
B      15.198 0.086  14.133 0.052  13.627 0.116  13.155 0.156 

C      13.317 0.093  12.673 0.058  12.316 0.121  11.980 0.161 

J      15.607 0.109  14.956 0.065  14.603 0.136  14.271 0.182  

K      16.573 0.123  15.975 0.082  15.547 0.174  15.147 0.231  


In the continuing saga of the dark current, I again found that dark images taken immediately after twilight sky flats -- even if no flatfield images were saturated -- showed higher counts than dark images taken at other times.

On this night, none of the twilight flats exceeded about 30,000 counts -- but the excess charge still accumulated. In the graph below, twilight flatfields were taken at time 47200, between the first and second set of darks.

This "ghost" or "excess" charge dissipates in a roughly exponential manner, with a time constant of about 600 seconds = 10 minutes.

Moral of the story: Wait about 40-60 minutes after flats before doing just about anything else.

I took sets of 15-20 images in each filter, guiding in VRI, but not B. I used longer guide exposure times in I. I discarded any trailed images.

As explained in the notes to Jun 14, 2016, I used the "rotsub" technique to remove the galaxy's light at the position of the SN.

On this night, I used "method 2", which means performing "rotsub" on each individual image, then combining all the resulting images in a passband to make a "master rotsub" image.

Using aperture photometry with a radius of 4 pixels (radius of 5.5 arcsec) I measured the instrumental magnitudes of a number of reference stars and the target. Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame. I used the interim reference magnitudes above plus color terms which I am currently revising -- so please treat these results as preliminary to convert the ensemble instrumental magnitudes to the standard Johnson-Cousins BVRI scale.

Note that in the graph below, I combine data calibrated with UCAC4 photometry (first few weeks) with recent data calibrated with AAVSO photometry. That's inconsistent, and I'll re-compute all magnitudes later. Note further that I use only 2 AAVSO stars (B and C) for calibration, for consistency with earlier measurements; I'll use additional stars in my final calculations.

     filter  mag         mag_uncert                          Julian Date

   SN  B =   16.003   +/-   0.082  (ens  0.075 zp  0.032)    2457580.60486 
   SN  V =   14.827   +/-   0.041  (ens  0.040 zp  0.009)    2457580.59557 
   SN  R =   14.473   +/-   0.103  (ens  0.034 zp  0.098)    2457580.58881 
   SN  I =   14.218   +/-   0.115  (ens  0.047 zp  0.105)    2457580.61869 

Below is a preliminary light curve, based on RIT Observatory measurements. I also show measurements of SN 2011fe in M101, an ordinary type Ia supernova, shifted arbitrarily.

Last modified 07/11/2016 by MWR.